In a number of industrial processes it is desirable to enhance the mass transfer of a gas into a liquid. Much of this need results from biochemical oxidation processes which use aerobic microbes. Aerobic microbes are employed because they are able to convert a raw material into a higher value material. Some examples include aerobic fermentation processes for manufacturing fragrances, flavors, and pharmaceutical components. Perhaps an even more important process is the aeration of sewage and other wastewater streams. What all these processes using aerobic microbes have in common is the need for oxygen to be dissolved into the liquid in order for the microbes to be able to convert the raw material into the desired result. Since the microbes work most efficiently when there is an adequate level of dissolved oxygen available in the liquid, it is typically desirable to transfer additional amounts of oxygen or air into the liquid. This can be accomplished in a number of ways but the two most common techniques are gas sparging and surface aeration. In a gas sparging procedure, a gas (e.g. air or oxygen) is bubbled through the liquid in a manner that increases the amount of dissolved oxygen in the liquid. In contrast, surface aeration uses an impeller located at the surface of the liquid to agitate or spray the liquid into the gas. The spray subsequently impinges on the liquid surface which also entrains gas into the liquid surface.
One of the basic procedures for the treatment of sewage and other wastewater streams is known as the activated sludge process, which was discovered or invented more than seventy years ago. It is a biochemical type of reaction, involving the mass transfer of oxygen into water, and then the transfer and use of that dissolved oxygen to support the growth of aerobic microorganisms suspended in the water. These microorganisms, known as the biomass, oxidize the organic materials in the wastewater in different ways to eliminate the biochemical oxygen demand (BOD) of the wastewater.
The original activated sludge process involved introducing air from a blower through various forms of diffuser devices located in the bottom of the aeration tank or basin. These devices generally have low oxygen-transfer efficiency and poor solids-suspension characteristics. More than forty years ago, a different approach was taken to aeration in the activated sludge process. This different approach was known as mechanical surface aeration. This technique made use of a mechanical agitator operating at the liquid surface to throw or spray liquid into the air and to induce entrainment of air into the liquid surface, without the use of a compressor and the diffusers. Since that time, a fairly large number of different designs for surface aeration impellers have been introduced, both for the purpose of increasing the oxygen-transfer efficiency and also, secondarily, if possible, to improve liquid mixing and solids suspension. The problem of solids suspension, however, has an obvious limitation because of the remoteness of the surface aeration impeller from the tank bottom where the biomass solids tend to settle if the bulk liquid in the tank is not adequately mixed.
The standard measure of aeration efficiency is the number of pounds of oxygen transferred into the liquid per hour per horsepower of energy used to operate the aeration system. This measure is known as the Standard Aeration Efficiency (SAE). The SAE for current state of the art surface aeration devices ranges from about 2.0 to about 3.3 pounds of oxygen per hour per horsepower in the larger aerator sizes. In smaller sizes, the efficiency values can be somewhat higher. Since wastewater treatment plants are pure cost centers (i.e. they do not sell a product) and since electric power is one of the main operating costs in such a plant, the oxygen-transfer efficiency performance of such aerators is extremely important, especially in the larger plants. This need has led to a number of attempts at producing surface aeration impeller designs with greater oxygen transfer efficiency.
Typical of state of the art surface aeration impellers are those shown in U.S. Pat. No. 3,479,017 to Thikotter; U.S. Pat. Nos. 3,576,316 and 3,610,590 to Kaelin; and U.S. Pat. No. 3,741,682 to Robertson; U.S. Pat. No. 4,066,383 to Lakin; U.S. Pat. No. 4,074,953 to Budde et al.; U.S. Pat. No. 4,151,231 to Austin; U.S. Pat. No. 4,334,826 to Connolly et al.; U.S. Pat. No. 5,522,989 to Hove; and U.S. Pat. No. 5,988,604 to McWhirter. All of these patents are incorporated herein in their entirety.
Thikotter discloses a surface aeration impeller for use in an activated sludge process. Thikotter's aerator comprises a flat, circular impeller disc having a plurality of impeller blades depending from the undersurface of the disc. The blades are generally flat, positioned radially and have a height that decreases from its inner edge to its outer edge. This design principally focuses on spraying the liquid and does not provide much up-pumping action or mixing of the tank liquid content resulting in relatively low efficiency of the system. Robertson and Austin also disclose surface aeration impellers having multiple blades located on the underside of a disc. Their blades are radial or approximately radial and generally flat but have a horizontal plate secured to the lower edge of each blade. Again, these designs primarily focus on throwing or spraying of the liquid and do not provide much up-pumping action and mixing of the body of liquid in the tank.
Unlike Thikotter, Roberston, and Austin, Lakin and Connolly disclose various forms of surface aeration impellers having primarily vertically curved blades. Most seem to have multiple blades on a disc-shaped mounting member. Kaelin and Budde et al. also teach surface aerator designs. The blades of Budde et al. are radial and Kaelin show other designs representative of the state of the art. The design of Budde et al. does not provide much mixing action and Kaelin in addition suffers from the disadvantage of being difficult to manufacture.
Hove teaches a device and method for aerating wastewater. The device has multiple blades positioned on a disc-shaped mounting member. The blades appear to be entirely radial. Hove's blades are unique compared with the above patents in that they are located both above and below the disc-shaped mounting member.
McWhirter '604 teaches a surface aeration impeller that is an axial flow impeller that may have either pitched blade turbine or airfoil shaped blades. The blades of the McWhirter patent are not mounted to the underside of a disc-shaped mounting member Additionally, while the upper section of the '604 blades are not strictly radial, the lower section is radial (at least at one point). This impeller does provide some up-pumping and mixing action but still leaves room for improved liquid pumping and oxygen transfer efficiency.
Although such surface aeration devices as discussed above have functioned in a generally satisfactory manner, problems have been experienced with excessive splashing and misting, insufficient liquid pumping, mixing and circulation, and clogging of the impellers during operation. Accordingly, there continues to be a need for improved designs that increase the efficiency of the aeration process and/or address some of these problems. In particular, surface aeration impeller designs and operational characteristics that increase the oxygen transfer efficiency into the liquid and thereby reduce operating costs are especially desirable.
Many of the limitations associated with prior art surface aerator impeller designs result from an insufficient understanding of the fundamental mechanisms and fluid dynamics of surface aeration. The current state-of-the-art oxygen mass transfer analysis for surface aerators is essentially limited to the simple, idealized model employed in the ASCE Standard for the Measurement of Oxygen Transfer in Clean Water. This oversimplified and limited model has been used for decades to characterize the oxygen mass transfer performance of surface aerators. A more realistic and rigorous model has been developed by McWhirter et al. in “Oxygen Mass Transfer Fundamentals of Surface Aerators”, Ind. Eng. Chem. Res. 34, 2644-2654, 1995. This mechanistic model provides a more physically realistic description of the actual oxygen transfer mechanisms of surface aerators and separates the oxygen mass transfer process into two distinct zones: a liquid spray mass transfer zone and a surface reaeration mass transfer zone.
These two distinctly different mechanisms or zones are created by all generic types of mechanical surface aerators. The liquid spray mass transfer zone 11 is created in the immediate gas space surrounding the periphery of the surface aeration impeller where the liquid is discharged into the surrounding gas at high velocity. The surface reaeration mass transfer zone 13 exists primarily outside the spray umbrella and in the bulk liquid near the surface in the area that is circumferential to the periphery of the liquid spray mass transfer zone. The two zones are indicated in FIG. 4. The liquid spray mass transfer zone can be reasonably characterized and modeled as a single-stage gas-liquid contacting zone wherein the liquid is dispersed into a virtually infinite, continuous gas phase of constant gas composition above the liquid surface. In contrast, the mechanism in the surface reaeration mass transfer zone is predominately characterized by oxygen transfer to a highly turbulent liquid surface containing entrained gas from the gas phase above the liquid surface. As the liquid spray zone impinges on the liquid surface of the tank, substantial gas bubble entrainment into the surface is accomplished and a “white-water” effect is produced at the periphery of the liquid spray impingement on the surface of the tank liquid. The surface reaeration mass transfer zone also includes the oxygen transfer to the highly turbulent liquid surface beneath the spray umbrella and thus includes all oxygen transfer to the surface liquid due to bubble entrainment and contact of the highly turbulent liquid surface with the gas above the liquid surface.
In contract to generally perceived prior opinion regarding the primary oxygen transfer mechanism of surface aerators, the present inventors have quantitatively shown that about two-thirds of the oxygen transfer of surface aerators occurs in the surface reaeration mass transfer zone and only about one-third in the liquid spray mass transfer zone. This suggests that impeller designs that enhance oxygen transfer in the surface reaeration zone (e.g. by increasing turbulence and volume flow rates) may have a greater overall effect on the total oxygen transfer of the system than impeller designs that focus primarily on increasing oxygen transfer in the spray zone (e.g. by improving spray characteristics like height and distance). Thus, a greater understanding of the oxygen mass transfer mechanisms in surface aerators has allowed the present inventors to independently analyze the oxygen transfer process within these two distinctively separate mass transfer zones leading to the improved surface aerator impeller designs as disclosed in this application. These new designs pump more liquid per unit of horsepower input through the liquid spray mass transfer zone and into the surface reaeration zone and thereby maximize the total oxygen mass transfer efficiency of the overall surface aeration system.
Accordingly, the following are selected objects of various embodiments of the present invention:
It is an object of the present invention to provide an improved surface aeration impeller having improved gas transfer rates into the liquid particularly in the surface reaeration mass transfer zone of the system.
It is also an object of the present invention to enhance turbulence and gas entrainment at the liquid surface created by the liquid spray of a surface aeration system.
It is an object of the present invention to provide an improved surface aeration impeller having reduced torque and increased rotational speed leading to reduced costs for motor and gear transmission equipment to rotate the impeller.
It is also an object of the present invention to provide an improved impeller design having increased liquid pumping capacity and efficiency.